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Experimental investigation of liquid holdup in structured packings
chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Experimental investigation of liquid holdup in structured packings Ali Zakeri a , Aslak Einbu b , Hallvard F. Svendsen a,∗ a b
Department of Chemical Engineering, NTNU, N-7491 Trondheim, Norway SINTEF Materials and Chemistry, N-7465 Trondheim, Norway
a b s t r a c t Liquid holdup is an important hydrodynamic parameter for characterizing the gas/liquid flow pattern in packed beds. In this paper, a study of liquid holdup in 3 different structured packings: Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC, and Montz-Pak B1-250M is presented, using air/water, air/water/sugar solutions with liquid viscosity up to 12 cP and air/30 wt% MEA in a 0.5 m ID absorption column with a packing height of 5 m. As expected, at a given liquid load, the liquid holdup was close to constant as a function of gas flow, with an increase at high gas velocities. In general, the Sulzer packing had a higher liquid holdup than observed in the two other packings. A possible explanation for this could be the lack of enhanced draining of liquid as seen with the modifications of the end-section of the Koch-Glitsch and Montz packings. Liquid holdup was found to increase with increasing liquid viscosity. The influence was higher at high liquid load than at low liquid load. Our results indicate a higher dependency at high liquid load and a lower at low liquid load. There was a reasonable agreement between our results and the data found in the literature. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: CO2 capture; Structured packing; Liquid holdup
1.
Introduction
Removal of CO2 as a greenhouse gas from flue gas stream is of vital importance. Anthropogenic sources of CO2 stem from e.g. combustion of fossil fuels and other industrial sources. The most viable technology for removing CO2 from flue gases is absorption by reactive absorbents (Hessen et al., 2010). Packed columns are common as gas/liquid contacting units for absorption processes and have maintained an important role in the process industries. Packed columns compared to tray columns are preferred where a high separation performance, low pressure drop and low liquid loads are required. In order to improve the technical tools for design and operation, a better understanding of the packing characteristics is necessary. The most important parameters in the hydrodynamics of packed columns are the pressure drop, mass transfer effective area and liquid holdup in addition to the mass transfer coefficient (Ataki, 2006). Several studies have been performed to measure liquid holdup in packed towers, however, most of them
∗
have focused on distillation (Ellenberger and Krishna, 1999; Muzen and Cassanello, 2005; Ratheesh and Kannan, 2004) and random packings, and the correlations obtained may not be directly applicable to reactive absorption. The aim of the present work is to perform measurements of liquid holdup in reactive absorbent systems. In this paper, results for liquid holdup are presented for 3 different structured packings: Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC, and Montz-Pak B1-250M using air/water, air/water/sugar solutions with liquid viscosity up to 12 cP and air/MEA in 0.5 m ID and 5 m high packing.
2.
Liquid holdup
Liquid holdup is an important hydrodynamic parameter for the gas–liquid contact and liquid side reactions in packed beds. It affects the pressure drop in the packing through the fluid effective velocity in the packing (Iliuta and Larachi, 2001) and also effects the lateral mixing in the packing liquid and
Corresponding author. Tel.: +47 73594100; fax: +47 73594080. E-mail addresses: [email protected] (A. Zakeri), [email protected] (A. Einbu), [email protected] (H.F. Svendsen). Received 20 May 2011; Received in revised form 5 August 2011; Accepted 10 August 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.08.012
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chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
Nomenclature a D F-factor g h hl hl,cal hl,dyn hl,mes hl,st Ql Red s ul ug ug,s ug,fl vb vd vf vl vp vs vt
specific surface area, m2 /m3 diameter of liquid droplet, m ug (g )0.5 , Pa0.5 gravitational constant, 9.81 m/s2 height of packing, m total liquid holdup, m3 /m3 calculated liquid holdup, m3 /m3 dynamic liquid holdup, m3 /m3 measured liquid holdup, m3 /m3 static liquid holdup, m3 /m3 liquid flow rate, m3 /s Reynolds number, Dvt p /L cross-sectional area of column, m2 liquid velocity, m/s superficial gas velocity, m/s loading gas velocity, m/s flooding gas velocity, m/s bed volume, m3 liquid distributor volume, m3 droplet volume in free spaces, m3 total liquid volume, m3 liquid present in piping, m3 sump volume, m3 terminal velocity, m/s
Fig. 1 – Relationship between the liquid holdup and the relative gas load.
Greek letter liquid fraction ˛d ε bed void fraction, m3 /m3 L liquid viscosity, cP surface tension, mN/m standard deviation, – i gas density, kg/m3 g p (L ) liquid density, kg/m3
Fig. 2 – Process flow diagram of the absorber.
thereby the availability of fresh interface. The liquid holdup is also used to design support devices for the column since it gives the liquid weight in operation (Suess and Spiegel, 1992). The relationships given in the literature for liquid holdup in packed columns apply merely to a few conventional forms of packing and have often been derived solely from measurements on air/water systems (Billet, 1995). The geometry of structured packings differs greatly, with the result that the existing relationships may not be expected to give accurate results for the prediction of liquid holdup in reactive absorption systems. Liquid holdup can be defined as the volume of stationary liquid that exists in the form of a film on the surface of the packing or is present in voids, dead spots, and phase boundaries in the bed of packing within a two-phase countercurrent column during the period in which the liquid phase descends at a constant rate onto the surface of the bed (Billet, 1995). There are two components of liquid holdup: a static hl,st and a dynamic hl,dyn . Thus hl = hl,st + hl,dyn
(1)
The dynamic component flows downwards, and the static basically remains within the bed, although some interaction between the two of course will take place. As the liquid load increases, the difference between the two components
becomes progressively greater. Fig. 1 visualizes the relationship between liquid holdup and gas load for a given liquid load. For low gas loads there is hardly any effect of increasing gas velocity on the liquid holdup. As the gas load increases for a given liquid load, the loading point of the bed is reached and the liquid holdup starts increasing when liquid velocity at the gas/liquid interface becomes zero. The loading point is located when the gas load is equal to ug,S in Fig. 1. Between the loading and flooding points in two-phase countercurrent flow, the downward stream of liquid is no longer independent of the gas load, because it is held up by the shear forces in the gas stream. Physically the pressure drop increases rapidly and the liquid starts exiting over the top of the column at the flooding point. Normally, flooding represents the maximum capacity condition of a packed column.
3.
Experimental setup and pilot plant
The experimental apparatus is presented in detail in Zakeri et al. (2010) so only a brief outline is given here. The process flow diagram is given in Fig. 2 showing the absorber test rig with a total height of 18 m. The pilot absorber column has 0.5 m ID, a packing height of 5 m and is designed for superficial gas velocities up to 5 m/s and a maximum liquid flow of 60 m3 /m2 h. The absorber fan has a capacity
chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
587
Fig. 3 – Liquid holdup (%) as function F-factor in the column for the packings (a) Mellapak 2X (Zakeri et al., 2010), (b) Flexipac 2Y HC and (c) B1-250M packings at different liquid loads (m3 /m2 h). Experiments with water/air.
Fig. 4 – Liquid holdup as function of F-factor for the different packings at low liquid load. (a) Mellapak 2X (5.7 m3 /m2 h) (Zakeri et al., 2010), (b) Flexipac 2Y HC (5 m3 /m2 h) and (c) B1-250M (5 m3 /m2 h).
Fig. 5 – Liquid holdup as function of F-factor for the different packings at 25 m3 /m2 h liquid load. (a) Mellapak 2X (Zakeri et al., 2010), (b) Flexipac 2Y HC and (c) B1-250M.
Fig. 6 – Liquid holdup as function of F-factor for the different packings at 5 and 25 m3 /m2 h liquid load. (a) Air/water, (b) air/sugar/water 5 cP and (c) air/sugar/water 12 cP.
of 3500 m3 /h and a maximum pressure head of 5.0 kPa. The 5 m packed column section is exchangeable and enables testing of different packing materials. The liquid levels in the absorber sump and the liquid distributor were monitored automatically by differential pressure sensors from Fuji Electric with high accuracy (1–10 mbar). The liquid distributor was a SulzerVKR2M channel-type distributor with 20 drip-points and a distribution density of 80 holes/m2 . In order to obtain
adequate straight sections for gas velocity measurements in the absorber, the main gas channel is laid out horizontally at the top of the tower. The liquid holdup in the packed bed was measured by adding a fixed amount of liquid to the absorber sump after drying the packing well (48 h with warm dry air). The liquid was then circulated from the sump over the column. Liquid holdup in the packing was determined by performing a mass
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chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
4.
Fig. 7 – Liquid holdup as function of F-factor for the different packings at 5 and 25 m3 /m2 h liquid loads. Experiments with 30 wt% MEA/air system.
balance of the total liquid added to the system by the following equation:
hl =
(vl − vs − vd − vp − vf ) × 100 vb
(2)
Liquid volumes in the sump (vs ) and in the liquid distributor (vd ) were determined by a FKC differential pressure transmitter sensors from Fuji Electric with high accuracy 1–10 mbar (1–4 mm). Volume of liquid in the free spaces (vf ) was calculated through the following equations:
2 < Red < 500 vt = 0.072
D1.6 (p − g )g
1/1.4 (3)
0.4 0.6 g g
0.00001 < Red < 2 vt = 0.0556
D2 (p − g )g g
(4)
Ql = ˛d · vt · s
(5)
vf = ˛d · h
(6)
The volume of liquid in the free spaces was found to vary from 2 to 10 l, and can be compared to 40 l to 180 l respectively of liquid in the packing bed. This constitutes a constant value of about 5% and has been subtracted in the holdup calculations. This setup is able to measure holdup with a total error of ±5%. Experiments were performed with 3 different structured packings Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC and Montz-Pak B1-250M. Air/water, air/water/sugar solutions with liquid viscosity up to 12 cP and air/MEA have been used for liquid holdup measurements.
Results and discussion
Fig. 3 shows results for measured liquid holdup (in percentage of the total packing volume m3 /m3 ) as a function of superficial gas velocity in the column at different liquid loads for the three different packing materials tested in experiments with the water and air system. As expected, at a given liquid load, the liquid holdup was close to constant as function of gas flow for the low gas velocities. For the Sulzer Mellapak 2X there is hardly any increase in holdup up to a F-factor of 2.5 Pa0.5 . For the other two packings a small increase can be seen in this pre-loading zone. Once loading is reached, the holdup increases more rapidly for Mellapak 2X and for the higher liquid loads, flooding is rapidly reached. For the other packings there is a more gradual increase in holdup with F-factor in the loading range. At a typical liquid load of 10 m3 /m2 h, the Sulzer packing reaches the loading region at F-factors above 3.0 Pa0.5 . The packing from Montz reaches the loading region above 3.2 Pa0.5 and the packing from Koch-Glitsch at above 3.4 Pa0.5 . In general, the Mellapak 2X has a higher liquid holdup than observed in the two other packings. This might be surprising as, taking into account the steeper angle of the Mellapak 2X, the opposite may have been more expected. A possible explanation for this could be the lack of enhanced draining of liquid as seen with the modifications of the end-section of the Koch-Glitsch and Montz packings. Another possibility is differences in corrugation withholding more liquid in the Mellapak 2X case. Figs. 4 and 5 show the liquid holdup (%) as a function of gas superficial velocity for 5 and 25 m3 /m2 h liquid loads for viscosities ranging between 1 and 12 cP using sucrose as viscosifier. In addition data for 30 wt% MEA are shown. Liquid holdup was generally found to increase significantly with increasing liquid viscosity as seen from Figs. 4 and 5. However, the effect varies with packing type and liquid load. At 5 m3 /m2 h for Mellapak 2X, water and 30 wt% MEA behave almost identically. The viscosity of 30 wt% MEA is close to 2.5 cP, but it has lower interfacial tension. Increasing the viscosity to 12 cP increases the holdup by about 50% for the lower gas loads, whereas the effect is weaker at the higher gas loads. It seems that increasing viscosity smoothens the transition between loading and flooding and to some extent lengthens the loading zone. The influence is higher at high liquid load than at low liquid load. Our results indicate a higher dependency at high liquid load and a lower at low liquid load. Regarding the maximum holdup observed for the three packings, in the Sulzer packing we could reach flooding-like behavior for all liquids tested. The maximum liquid holdup observed in the Sulzer packing is around 13% for all liquid
Fig. 8 – Liquid holdup measured and liquid holdup from literature vs. F-factor in the column for the (a) Flexipac and (b) Montz B1-250M.
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chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
systems tested. The two other packings from Koch-Glitsch and Montz showed a gradual increase in maximum holdup observed as a function of liquid viscosity. For the Montz packing, the highest liquid holdup observed during the experiments was about 14% and for the Koch-Glitsch packing it was 13%. The reason for these results is that the full capacity of the pressure head of the absorber fan is reached at the maximum holdup of around 13% for the Sulzer packing during floodinglike conditions in the column with excessive liquid build-up of liquid in the lower part of the column. The Montz-Pak B1-250M showed the lowest liquid holdup of the three packings. For a typical operational condition of 10 m3 /m2 h 30 wt% MEA and an F-factor of 2 m/s, Montz-Pak B1-250M showed a measured holdup of 6%. For the same conditions, Sulzer Mellapak 2X showed a liquid holdup of 8% and Koch-Glitsch Flexipac 2Y HC; 7.5%. In general, the Sulzer packing has a slightly higher liquid holdup than the two other packings at low liquid load, 5 m3 /m2 h, but the differences are not large, Fig. 6. The effect of increased viscosity is to increase the liquid holdup from around 3–4% for water to around 6–7% for 12 cP sugar solution. It is also clear from the shape of the curves that the flooding point is moved toward lower F-factors. This is reasonable as high viscosity means higher interaction forces are needed to reverse the liquid flow. At 25 m3 /m2 h the picture is more unclear. The Sulzer M2X starts out with higher holdups for water, but the increase with increasing viscosity is the lowest among the packings making M2X have the lowest holdup at 12 cP. The two other packings follow each other closely. In Fig. 7 data for 30 wt% MEA are shown. The viscosity of the MEA solution is close to water, but the interfacial tension and wetting properties are different. At the low liquid flow rate the difference from the water results is very small for all three packings. At 25 m3 /m2 h the data for M2X are about the same as for water, whereas for the two other packing there is an increase in holdup. This may have to do with the surface of the packing possibly being less affected by solvent properties for the M2X. However, any hard conclusions cannot be drawn. Data found in the literature for Koch-Glitsch Flexipac 2Y HC (Iliuta et al., 2004) and Montz-Pak B1-250M (Verschoof et al., 1999) were only for air and water system. They show reasonable agreement with our data as seen in Fig. 8. To make it more clear, standard deviations are given in Table 1. Fig. 9 shows comparison between our experimental data and Mellapak 250Y (Brunazzi and Paglianti, 1997); column diameter 0.5 m, Mellapak 250Y metal; column diameter 0.1 m, Mellapak 2X (Rochelle, 2010); column diameter 0.43 m and Mellapak 250X metal (Suess and Spiegel, 1992); column diameter 1 m; for air/water system.
Table 1 – Standard deviation for air and water system.
a% a
Koch-Glitsch-Flexipac 2Y HC
Montz-Pak B1-250M
±15%
±17%
i = (hl,measured − hl,lit. )/hl,measured × 100.
Table 2 – AD% and ADD% for Mellapak 2X.
ADa % ADDb %
Air/water
Air/water/ sugar 5 cP
Air/water/ sugar 12 cP
Air/MEA
18 11
41 36
29 25
25 22
Xi = (hl,mes − hl,cal )/hl,mes × 100. a b
|X |. i Xi X¯ . AAD = (1/N) AD = (1/N)
Fig. 9 – Liquid holdup measured and Liquid holdup from literature vs. F-factor in the column for the Mellapak 2X, Mellapak 250Y (Brunazzi and Paglianti, 1997) and Mellapak 250X (Suess and Spiegel, 1992).
5.
Comparisons with software
For the three packings only Sulzer provides software that can predict liquid holdup. These were calculated in SULCOL for Mellapak 2X for each liquid system. As Fig. 9 indicates, all our experimental data show higher liquid holdups than predicted by the software. For low liquid loads the difference is not large, but for water at 25 m3 /m2 h a difference of about 3% points is visible in the range of low F-factors. The holdup measured was, under these conditions, about 10% so the difference is significant. At higher F-factors the difference increases considerably, and it seems that the measured increase in holdup
Fig. 10 – Liquid holdup measured – liquid hold-up calculated from SULCOL vs. F-factor in the column for the packings Mellapak 2X at different liquid loads (m3 /m2 h). Experiments respectively with (a) air/water, (b) air/water sugar 5 cP, (c) air/water/sugar 12 cP and (d) MEA 30 wt%/air.
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chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
when flooding is approaching is not predicted by the software. The same general picture is shown for the other solvent systems (Fig. 10). The difference between experimental and predicted value does not change a lot with viscosity, but is highest for 5 cP sugar solution. The results for average deviation between and absolute average error liquid holdup for all systems and SULCOL software are presented in Table 2. The holdups predicted by the SULCOL software is based on direct measurements of liquid holdup in a column with 1 m ID, using gamma scan technology for air/water system (Suess and Spiegel, 1992). Measured data below the loading point form the basis for the SULCOL software and above loading point the data in SULCOL software are extrapolated values. The above figures show the small difference between our experimental data and SULCOL software predicted values below loading point whereas the differences becomes significant at and above the loading point.
6.
Conclusion
Results from a study of liquid holdup in 3 different structured packings; Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC, and Montz-Pak B1-250M, were presented. The fluid systems tested were: air/water, air/aqueous sugar solutions with liquid viscosities from 2.6 to 12 cP and finally air/30 wt% MEA. The column was 0.5 m ID and with 5 m packing height. An influence of viscosity on liquid holdup and flooding velocity was found. At a given liquid load, the holdup was close to constant as a function of gas flow, with a sharp increase at very high gas velocities. Liquid holdup was found to increase with increasing liquid viscosity. The data found in the literature for Koch-Gitsch Flexipac 2Y HC and Montz-Pak B1-250M were in a reasonable agreement with the measured data.
Acknowledgments Financial support provided from Statoil and the Research Council of Norway through the VOCC project, and Statoil, Shell Technology Norway AS, Metso Automation, Det Norske Veritas AS, and the Research Council of Norway through the CCERT project (182607), is greatly appreciated.
References Ataki, A., 2006. Wetting of Structured Packing Elements – CFD and Experiment, Faculty of Mechanical and Process Engineering Technical University of Kaiserslautern, Kaiserslautern, p. 139. Billet, R., 1995. Packed Towers. VCH Publishers, Inc., New York, USA. Brunazzi, E., Paglianti, A., 1997. Mechanistic pressure drop model for columns containing structured packings. AIChE Journal 43, 317–327. Ellenberger, J., Krishna, R., 1999. Counter-current operation of structured catalytically packed distillation columns: pressure drop, holdup and mixing. Chemical Engineering Science 54, 1339–1345. Hessen, E.T., Haug-Warberg, T., Svendsen, H.F., 2010. The refined e-NRTL model applied to CO2 –H2 O–alkanolamine systems. Chemical Engineering Science 65, 3638–3648. Iliuta, I., Larachi, F., 2001. Mechanistic model for structured packing containing column: irrigated pressure drop, liquid hold-up, end packing fractional wetted area. Ind. Eng. Chem. Res. 40, 4146–5140. Iliuta, I., Petre, C.F., Larachi, F., 2004. Hydrodynamic continuum model for two-phase flow structured-packing-containing columns. Chemical Engineering Science 59, 879–888. Muzen, A., Cassanello, M.C., 2005. Liquid holdup in columns packed with structured packings: countercurrent vs. cocurrent operation. Chemical Engineering Science 60, 6226–6234. Ratheesh, S., Kannan, A., 2004. Holdup and pressure drop studies in structured packings with catalysts. Chemical Engineering Journal 104, 45–54. Rochelle, G.T., 2010. CO2 Capture by Aqueous Absorption Summary of 4th Quarterly Progress Reports 2009. Department of Chemical Engineering, The University of Texas at Austin, Texas at Austin, p. 910. Suess, P., Spiegel, L., 1992. Holdup of Mellapack structured packings. Chemical Engineering and Processing 31, 119–124. Verschoof, H.-J., Olujic, Z., Fair, J.R., 1999. A general correlation for prediction the loading point of corrugated sheet structured packings. Industrial and Engineering Chemistry 38, 3663–3669. Zakeri, A., Einbu, A., Wiig, P.O., Øi, L.E., Svendsen, H.F., 2010. Experimental Investigation of Pressure Drop, Liquid Hold-Up and Mass Transfer Parameters in a 0.5 m Diameter Absorber Column, GHGT10, Amsterdam, The Netherlands.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Experimental investigation of liquid holdup in structured packings Ali Zakeri a , Aslak Einbu b , Hallvard F. Svendsen a,∗ a b
Department of Chemical Engineering, NTNU, N-7491 Trondheim, Norway SINTEF Materials and Chemistry, N-7465 Trondheim, Norway
a b s t r a c t Liquid holdup is an important hydrodynamic parameter for characterizing the gas/liquid flow pattern in packed beds. In this paper, a study of liquid holdup in 3 different structured packings: Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC, and Montz-Pak B1-250M is presented, using air/water, air/water/sugar solutions with liquid viscosity up to 12 cP and air/30 wt% MEA in a 0.5 m ID absorption column with a packing height of 5 m. As expected, at a given liquid load, the liquid holdup was close to constant as a function of gas flow, with an increase at high gas velocities. In general, the Sulzer packing had a higher liquid holdup than observed in the two other packings. A possible explanation for this could be the lack of enhanced draining of liquid as seen with the modifications of the end-section of the Koch-Glitsch and Montz packings. Liquid holdup was found to increase with increasing liquid viscosity. The influence was higher at high liquid load than at low liquid load. Our results indicate a higher dependency at high liquid load and a lower at low liquid load. There was a reasonable agreement between our results and the data found in the literature. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: CO2 capture; Structured packing; Liquid holdup
1.
Introduction
Removal of CO2 as a greenhouse gas from flue gas stream is of vital importance. Anthropogenic sources of CO2 stem from e.g. combustion of fossil fuels and other industrial sources. The most viable technology for removing CO2 from flue gases is absorption by reactive absorbents (Hessen et al., 2010). Packed columns are common as gas/liquid contacting units for absorption processes and have maintained an important role in the process industries. Packed columns compared to tray columns are preferred where a high separation performance, low pressure drop and low liquid loads are required. In order to improve the technical tools for design and operation, a better understanding of the packing characteristics is necessary. The most important parameters in the hydrodynamics of packed columns are the pressure drop, mass transfer effective area and liquid holdup in addition to the mass transfer coefficient (Ataki, 2006). Several studies have been performed to measure liquid holdup in packed towers, however, most of them
∗
have focused on distillation (Ellenberger and Krishna, 1999; Muzen and Cassanello, 2005; Ratheesh and Kannan, 2004) and random packings, and the correlations obtained may not be directly applicable to reactive absorption. The aim of the present work is to perform measurements of liquid holdup in reactive absorbent systems. In this paper, results for liquid holdup are presented for 3 different structured packings: Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC, and Montz-Pak B1-250M using air/water, air/water/sugar solutions with liquid viscosity up to 12 cP and air/MEA in 0.5 m ID and 5 m high packing.
2.
Liquid holdup
Liquid holdup is an important hydrodynamic parameter for the gas–liquid contact and liquid side reactions in packed beds. It affects the pressure drop in the packing through the fluid effective velocity in the packing (Iliuta and Larachi, 2001) and also effects the lateral mixing in the packing liquid and
Corresponding author. Tel.: +47 73594100; fax: +47 73594080. E-mail addresses: [email protected] (A. Zakeri), [email protected] (A. Einbu), [email protected] (H.F. Svendsen). Received 20 May 2011; Received in revised form 5 August 2011; Accepted 10 August 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.08.012
586
chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
Nomenclature a D F-factor g h hl hl,cal hl,dyn hl,mes hl,st Ql Red s ul ug ug,s ug,fl vb vd vf vl vp vs vt
specific surface area, m2 /m3 diameter of liquid droplet, m ug (g )0.5 , Pa0.5 gravitational constant, 9.81 m/s2 height of packing, m total liquid holdup, m3 /m3 calculated liquid holdup, m3 /m3 dynamic liquid holdup, m3 /m3 measured liquid holdup, m3 /m3 static liquid holdup, m3 /m3 liquid flow rate, m3 /s Reynolds number, Dvt p /L cross-sectional area of column, m2 liquid velocity, m/s superficial gas velocity, m/s loading gas velocity, m/s flooding gas velocity, m/s bed volume, m3 liquid distributor volume, m3 droplet volume in free spaces, m3 total liquid volume, m3 liquid present in piping, m3 sump volume, m3 terminal velocity, m/s
Fig. 1 – Relationship between the liquid holdup and the relative gas load.
Greek letter liquid fraction ˛d ε bed void fraction, m3 /m3 L liquid viscosity, cP surface tension, mN/m standard deviation, – i gas density, kg/m3 g p (L ) liquid density, kg/m3
Fig. 2 – Process flow diagram of the absorber.
thereby the availability of fresh interface. The liquid holdup is also used to design support devices for the column since it gives the liquid weight in operation (Suess and Spiegel, 1992). The relationships given in the literature for liquid holdup in packed columns apply merely to a few conventional forms of packing and have often been derived solely from measurements on air/water systems (Billet, 1995). The geometry of structured packings differs greatly, with the result that the existing relationships may not be expected to give accurate results for the prediction of liquid holdup in reactive absorption systems. Liquid holdup can be defined as the volume of stationary liquid that exists in the form of a film on the surface of the packing or is present in voids, dead spots, and phase boundaries in the bed of packing within a two-phase countercurrent column during the period in which the liquid phase descends at a constant rate onto the surface of the bed (Billet, 1995). There are two components of liquid holdup: a static hl,st and a dynamic hl,dyn . Thus hl = hl,st + hl,dyn
(1)
The dynamic component flows downwards, and the static basically remains within the bed, although some interaction between the two of course will take place. As the liquid load increases, the difference between the two components
becomes progressively greater. Fig. 1 visualizes the relationship between liquid holdup and gas load for a given liquid load. For low gas loads there is hardly any effect of increasing gas velocity on the liquid holdup. As the gas load increases for a given liquid load, the loading point of the bed is reached and the liquid holdup starts increasing when liquid velocity at the gas/liquid interface becomes zero. The loading point is located when the gas load is equal to ug,S in Fig. 1. Between the loading and flooding points in two-phase countercurrent flow, the downward stream of liquid is no longer independent of the gas load, because it is held up by the shear forces in the gas stream. Physically the pressure drop increases rapidly and the liquid starts exiting over the top of the column at the flooding point. Normally, flooding represents the maximum capacity condition of a packed column.
3.
Experimental setup and pilot plant
The experimental apparatus is presented in detail in Zakeri et al. (2010) so only a brief outline is given here. The process flow diagram is given in Fig. 2 showing the absorber test rig with a total height of 18 m. The pilot absorber column has 0.5 m ID, a packing height of 5 m and is designed for superficial gas velocities up to 5 m/s and a maximum liquid flow of 60 m3 /m2 h. The absorber fan has a capacity
chemical engineering research and design 9 0 ( 2 0 1 2 ) 585–590
587
Fig. 3 – Liquid holdup (%) as function F-factor in the column for the packings (a) Mellapak 2X (Zakeri et al., 2010), (b) Flexipac 2Y HC and (c) B1-250M packings at different liquid loads (m3 /m2 h). Experiments with water/air.
Fig. 4 – Liquid holdup as function of F-factor for the different packings at low liquid load. (a) Mellapak 2X (5.7 m3 /m2 h) (Zakeri et al., 2010), (b) Flexipac 2Y HC (5 m3 /m2 h) and (c) B1-250M (5 m3 /m2 h).
Fig. 5 – Liquid holdup as function of F-factor for the different packings at 25 m3 /m2 h liquid load. (a) Mellapak 2X (Zakeri et al., 2010), (b) Flexipac 2Y HC and (c) B1-250M.
Fig. 6 – Liquid holdup as function of F-factor for the different packings at 5 and 25 m3 /m2 h liquid load. (a) Air/water, (b) air/sugar/water 5 cP and (c) air/sugar/water 12 cP.
of 3500 m3 /h and a maximum pressure head of 5.0 kPa. The 5 m packed column section is exchangeable and enables testing of different packing materials. The liquid levels in the absorber sump and the liquid distributor were monitored automatically by differential pressure sensors from Fuji Electric with high accuracy (1–10 mbar). The liquid distributor was a SulzerVKR2M channel-type distributor with 20 drip-points and a distribution density of 80 holes/m2 . In order to obtain
adequate straight sections for gas velocity measurements in the absorber, the main gas channel is laid out horizontally at the top of the tower. The liquid holdup in the packed bed was measured by adding a fixed amount of liquid to the absorber sump after drying the packing well (48 h with warm dry air). The liquid was then circulated from the sump over the column. Liquid holdup in the packing was determined by performing a mass
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4.
Fig. 7 – Liquid holdup as function of F-factor for the different packings at 5 and 25 m3 /m2 h liquid loads. Experiments with 30 wt% MEA/air system.
balance of the total liquid added to the system by the following equation:
hl =
(vl − vs − vd − vp − vf ) × 100 vb
(2)
Liquid volumes in the sump (vs ) and in the liquid distributor (vd ) were determined by a FKC differential pressure transmitter sensors from Fuji Electric with high accuracy 1–10 mbar (1–4 mm). Volume of liquid in the free spaces (vf ) was calculated through the following equations:
2 < Red < 500 vt = 0.072
D1.6 (p − g )g
1/1.4 (3)
0.4 0.6 g g
0.00001 < Red < 2 vt = 0.0556
D2 (p − g )g g
(4)
Ql = ˛d · vt · s
(5)
vf = ˛d · h
(6)
The volume of liquid in the free spaces was found to vary from 2 to 10 l, and can be compared to 40 l to 180 l respectively of liquid in the packing bed. This constitutes a constant value of about 5% and has been subtracted in the holdup calculations. This setup is able to measure holdup with a total error of ±5%. Experiments were performed with 3 different structured packings Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC and Montz-Pak B1-250M. Air/water, air/water/sugar solutions with liquid viscosity up to 12 cP and air/MEA have been used for liquid holdup measurements.
Results and discussion
Fig. 3 shows results for measured liquid holdup (in percentage of the total packing volume m3 /m3 ) as a function of superficial gas velocity in the column at different liquid loads for the three different packing materials tested in experiments with the water and air system. As expected, at a given liquid load, the liquid holdup was close to constant as function of gas flow for the low gas velocities. For the Sulzer Mellapak 2X there is hardly any increase in holdup up to a F-factor of 2.5 Pa0.5 . For the other two packings a small increase can be seen in this pre-loading zone. Once loading is reached, the holdup increases more rapidly for Mellapak 2X and for the higher liquid loads, flooding is rapidly reached. For the other packings there is a more gradual increase in holdup with F-factor in the loading range. At a typical liquid load of 10 m3 /m2 h, the Sulzer packing reaches the loading region at F-factors above 3.0 Pa0.5 . The packing from Montz reaches the loading region above 3.2 Pa0.5 and the packing from Koch-Glitsch at above 3.4 Pa0.5 . In general, the Mellapak 2X has a higher liquid holdup than observed in the two other packings. This might be surprising as, taking into account the steeper angle of the Mellapak 2X, the opposite may have been more expected. A possible explanation for this could be the lack of enhanced draining of liquid as seen with the modifications of the end-section of the Koch-Glitsch and Montz packings. Another possibility is differences in corrugation withholding more liquid in the Mellapak 2X case. Figs. 4 and 5 show the liquid holdup (%) as a function of gas superficial velocity for 5 and 25 m3 /m2 h liquid loads for viscosities ranging between 1 and 12 cP using sucrose as viscosifier. In addition data for 30 wt% MEA are shown. Liquid holdup was generally found to increase significantly with increasing liquid viscosity as seen from Figs. 4 and 5. However, the effect varies with packing type and liquid load. At 5 m3 /m2 h for Mellapak 2X, water and 30 wt% MEA behave almost identically. The viscosity of 30 wt% MEA is close to 2.5 cP, but it has lower interfacial tension. Increasing the viscosity to 12 cP increases the holdup by about 50% for the lower gas loads, whereas the effect is weaker at the higher gas loads. It seems that increasing viscosity smoothens the transition between loading and flooding and to some extent lengthens the loading zone. The influence is higher at high liquid load than at low liquid load. Our results indicate a higher dependency at high liquid load and a lower at low liquid load. Regarding the maximum holdup observed for the three packings, in the Sulzer packing we could reach flooding-like behavior for all liquids tested. The maximum liquid holdup observed in the Sulzer packing is around 13% for all liquid
Fig. 8 – Liquid holdup measured and liquid holdup from literature vs. F-factor in the column for the (a) Flexipac and (b) Montz B1-250M.
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systems tested. The two other packings from Koch-Glitsch and Montz showed a gradual increase in maximum holdup observed as a function of liquid viscosity. For the Montz packing, the highest liquid holdup observed during the experiments was about 14% and for the Koch-Glitsch packing it was 13%. The reason for these results is that the full capacity of the pressure head of the absorber fan is reached at the maximum holdup of around 13% for the Sulzer packing during floodinglike conditions in the column with excessive liquid build-up of liquid in the lower part of the column. The Montz-Pak B1-250M showed the lowest liquid holdup of the three packings. For a typical operational condition of 10 m3 /m2 h 30 wt% MEA and an F-factor of 2 m/s, Montz-Pak B1-250M showed a measured holdup of 6%. For the same conditions, Sulzer Mellapak 2X showed a liquid holdup of 8% and Koch-Glitsch Flexipac 2Y HC; 7.5%. In general, the Sulzer packing has a slightly higher liquid holdup than the two other packings at low liquid load, 5 m3 /m2 h, but the differences are not large, Fig. 6. The effect of increased viscosity is to increase the liquid holdup from around 3–4% for water to around 6–7% for 12 cP sugar solution. It is also clear from the shape of the curves that the flooding point is moved toward lower F-factors. This is reasonable as high viscosity means higher interaction forces are needed to reverse the liquid flow. At 25 m3 /m2 h the picture is more unclear. The Sulzer M2X starts out with higher holdups for water, but the increase with increasing viscosity is the lowest among the packings making M2X have the lowest holdup at 12 cP. The two other packings follow each other closely. In Fig. 7 data for 30 wt% MEA are shown. The viscosity of the MEA solution is close to water, but the interfacial tension and wetting properties are different. At the low liquid flow rate the difference from the water results is very small for all three packings. At 25 m3 /m2 h the data for M2X are about the same as for water, whereas for the two other packing there is an increase in holdup. This may have to do with the surface of the packing possibly being less affected by solvent properties for the M2X. However, any hard conclusions cannot be drawn. Data found in the literature for Koch-Glitsch Flexipac 2Y HC (Iliuta et al., 2004) and Montz-Pak B1-250M (Verschoof et al., 1999) were only for air and water system. They show reasonable agreement with our data as seen in Fig. 8. To make it more clear, standard deviations are given in Table 1. Fig. 9 shows comparison between our experimental data and Mellapak 250Y (Brunazzi and Paglianti, 1997); column diameter 0.5 m, Mellapak 250Y metal; column diameter 0.1 m, Mellapak 2X (Rochelle, 2010); column diameter 0.43 m and Mellapak 250X metal (Suess and Spiegel, 1992); column diameter 1 m; for air/water system.
Table 1 – Standard deviation for air and water system.
a% a
Koch-Glitsch-Flexipac 2Y HC
Montz-Pak B1-250M
±15%
±17%
i = (hl,measured − hl,lit. )/hl,measured × 100.
Table 2 – AD% and ADD% for Mellapak 2X.
ADa % ADDb %
Air/water
Air/water/ sugar 5 cP
Air/water/ sugar 12 cP
Air/MEA
18 11
41 36
29 25
25 22
Xi = (hl,mes − hl,cal )/hl,mes × 100. a b
|X |. i Xi X¯ . AAD = (1/N) AD = (1/N)
Fig. 9 – Liquid holdup measured and Liquid holdup from literature vs. F-factor in the column for the Mellapak 2X, Mellapak 250Y (Brunazzi and Paglianti, 1997) and Mellapak 250X (Suess and Spiegel, 1992).
5.
Comparisons with software
For the three packings only Sulzer provides software that can predict liquid holdup. These were calculated in SULCOL for Mellapak 2X for each liquid system. As Fig. 9 indicates, all our experimental data show higher liquid holdups than predicted by the software. For low liquid loads the difference is not large, but for water at 25 m3 /m2 h a difference of about 3% points is visible in the range of low F-factors. The holdup measured was, under these conditions, about 10% so the difference is significant. At higher F-factors the difference increases considerably, and it seems that the measured increase in holdup
Fig. 10 – Liquid holdup measured – liquid hold-up calculated from SULCOL vs. F-factor in the column for the packings Mellapak 2X at different liquid loads (m3 /m2 h). Experiments respectively with (a) air/water, (b) air/water sugar 5 cP, (c) air/water/sugar 12 cP and (d) MEA 30 wt%/air.
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when flooding is approaching is not predicted by the software. The same general picture is shown for the other solvent systems (Fig. 10). The difference between experimental and predicted value does not change a lot with viscosity, but is highest for 5 cP sugar solution. The results for average deviation between and absolute average error liquid holdup for all systems and SULCOL software are presented in Table 2. The holdups predicted by the SULCOL software is based on direct measurements of liquid holdup in a column with 1 m ID, using gamma scan technology for air/water system (Suess and Spiegel, 1992). Measured data below the loading point form the basis for the SULCOL software and above loading point the data in SULCOL software are extrapolated values. The above figures show the small difference between our experimental data and SULCOL software predicted values below loading point whereas the differences becomes significant at and above the loading point.
6.
Conclusion
Results from a study of liquid holdup in 3 different structured packings; Mellapak 2X from Sulzer, Koch-Glitsch Flexipac 2Y HC, and Montz-Pak B1-250M, were presented. The fluid systems tested were: air/water, air/aqueous sugar solutions with liquid viscosities from 2.6 to 12 cP and finally air/30 wt% MEA. The column was 0.5 m ID and with 5 m packing height. An influence of viscosity on liquid holdup and flooding velocity was found. At a given liquid load, the holdup was close to constant as a function of gas flow, with a sharp increase at very high gas velocities. Liquid holdup was found to increase with increasing liquid viscosity. The data found in the literature for Koch-Gitsch Flexipac 2Y HC and Montz-Pak B1-250M were in a reasonable agreement with the measured data.
Acknowledgments Financial support provided from Statoil and the Research Council of Norway through the VOCC project, and Statoil, Shell Technology Norway AS, Metso Automation, Det Norske Veritas AS, and the Research Council of Norway through the CCERT project (182607), is greatly appreciated.
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